The measurement of the heating value of natural gas is important in controlling combustion and is a necessary measurement in the distribution and sale of natural gas. There are four useful methods for measuring heating value.
The first method for measuring heating value is calorimetric measurement in which a volume of the gas is combusted. An amount of heat is liberated by the complete combustion and is carefully accumulated and measured. The amount of heat liberated is manifested by a change in temperature. This method is the original method employed and usually requires extreme control of flows and temperatures. The apparatus usually requires extensive maintenance.
The second method for measuring heating value is constituent analysis. Using a gas chromatograph, the fraction of each chemical constituent in the gas is determined. Then heating value is determined by summing the heating values for the individual constituents according to their fractional presence. The problem with constituent analysis is the reliability of the apparatus and its linearity. Gas chromatographs require constant maintenance and have a limited range for heating value measurement unless calibrated with a reference gas that is very similar to the sample gas.
The third method is stoichiometry, in which combustion is substantially completed with a perfect amount of oxygen. In this case, natural gases are combusted with air and the fuel-to-air ratio is adjusted until combustion results in either a maximum flame temperature or the stoichiometric point of perfect combustion, i.e., the knife edge when there is no remaining oxygen.
Clingman, U.S. Pat. No. 3,777,562, is an example of the third method. In Clingman, heating value is measured by combustion of a gas with amounts of air that are adjusted to obtain the maximum flame temperature. This is further disclosed in Clingman, U.S. Pat. Nos. 4,062,236, 4,125,018 and 4,125,123. In each of these patents, the combustion of the air-gas mixture is accomplished with a combustion flame on a burner top and with a temperature sensing device such as a thermocouple. Certain environments cannot be served by equipment that presents an open flame.
The fourth method utilizes catalytic combustion. Gas is passed over a heated catalyst and oxidized. The amount of heat liberated can be measured either by temperature changes related to the catalytic reaction, by changes in power supplied to heat the catalyst or by measuring the temperature of the catalytic material. Catalytic combustion or catalytic oxidation is a known phenomenon with hydrocarbons. A mixture of hydrocarbon gas and air in the presence of platinum and/or palladium material will produce an oxidation reaction. The reaction occurs at temperatures below the auto-ignition temperature associated with a hydrocarbon. For example, methane when mixed with air will ignite at a temperature of about 730.degree. C. and reach an open flame at a temperature exceeding 1600.degree. C. Catalytic oxidation can take place at catalyst temperatures as low as 400.degree. C. although efficient catalytic activity is achieved at temperatures near 500.degree.-600.degree. C.
One problem with catalytic oxidation is the potential for poisoning the catalyst. Certain chemicals such as sulfur or lead and numerous others, can combine with and disable a catalyst and therefore eliminate its usefulness in heating value measurement. In many processes, such as land fill gas recovery, the gases contain "poisons" in sufficient quantity to have a high probability of disabling the measurement process.